I have a job and I need to do an underbase. I’ve been in the printing industry for 30 years but not tshirts. I simply need a knock out and the original designer used freaking Corel draw. Ugh 
I wish I could help, but I’ve never used CorelDRAW in my life. I do sympathize with you, though.
Thank you for your empathy. Archaic but still a staple in the garment printing industry (my boss is a cheapskate)
When you say Knockout, you mean a stencil to pull a white underspot?
Select all objects, copy them to a new layer, and in CorelDraw, use the Weld button in the top bar (it’s also in a menu somewhere.) That should give you a solid shape that incorporates all the other shapes. Clean up what needs cleaning up, like small holes that are too small to matter.
If you need a trap look under the Drop Shadow tool in the tool bar and use Contour and set your value to a negative number so it falls inside your shape… That’s about all I remember, CorelDraw was ages ago.
If the art is raster and you need to pull a white border, that’s all about the trace tool and I don’t remember anything about that.